Get UPN or email for logged in user in a .NET web application

I have an ASP. NET web application that makes user of the user credentials of the logged in user. Currently it uses the SID which comes from

System.Security.Principal.WindowsIdentity.GetCurrent().User.Value

I need to get either the users UPN login or email address (as defined in active directory) instead of the SID. GetCurrent() returns an object of type WindowsIdentity; looking in the details for WindowsIdentity Members:

I can't see anything that looks like it would give me either the UPN or email in there. How can I pull up that information to use, either by feeding the SID into some other function or calling something different in the first place.

System.Security.Principal.WindowsIdentity.GetCurrent().Name returns "DOMAIN\user", so that is not the answer.

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